1. Field of the invention
The invention relates to a device for stressed anchorage in rock.
Similar anchorages are known, notably for the retention of the arch of tunnels excavated in rock. They allow the bolted rock to be stabilized and to exercise a prestress on the walls of these tunnels, leading to the possibility of a very fast and very sure advance of the excavation works, due to the ease of fixing.
Over the years various methods have been developed for installing this type of anchorage.
2. Description of the prior art
In accordance with the imperatives to which they are subjected, it is noticed that the short type anchorages are usually no longer than one or two meters and the long type anchorages reaching ten meters and more.
The short type anchorages are usually of an elementary design but easy to handle. They are not designed for the application of real prestress tensions as their length is limited. Moreover, their principle of operation is mechanically inapplicable to long type anchorages. One of the oldest amongst the short type anchorages is the Perfo type bolt in which two perforated half-cylinders packed tight with mortar are coupled together and inserted in the hole, the mortar then being forced through the perforations by percussion. The known disadvantages of this system are due to its precarious results, because of the small volume of mortar which it contains and the insufficient available control of the conditions of anchorage.
The Selfix type tie rods comprise a dual compartment breakable capsule filled with two-constituent resin fitted firmly on a metal rod of standardized length. After the capsule has split open, a drilling machine is fitted on Latter designed to complete the mixing of the constituents. Such a principle is inapplicable to long anchorages because of the couple to be applied to the tie rod and of the distortion of the tie rod.
Furthermore, with this method, it is not possible to carry out an injection of mortar filling later on.
The homogeneity of the mixture and the evenness of the hardening are always problematical.
The SWELLEX anchorage is realized by expansion of a draw tube with compressed air, its principle is completely different from that of anchorages fixed in place by a synthetic cement.
All these anchorage devices apply only to limited lengths of tie rod and are not designed to exert a real prestress tension.
The long type anchorages, usually heavy duty, have a part sealed to the bottom of the drilling hole connected by tie rods to an anchorage head resting on the opening of this hole. Such anchorages enable several tens of tons of prestress tension to be exerted on the rock, the sealed part being ten meters or more distant from the anchorage head.
Most of these methods or devices for anchorage require either an injection or an introduction of sealing material before the tie rods are placed in position, which obviously doubles the handling operations necessary or the introduction of this sealing material after the anchorage device has been placed in position, a method requiring more time, as well as an elaborate design, more costly and the source of operational snags.
The Dywidag type anchorages use anchorage tie rods made of threaded bars partially sleeved with a sealing sleeve, the end of which is sealed with resin. The introduction of the sealing material is carried out before the placing in position of the anchorage tie rods; it is not possible with simultaneous check later on of the anchorage tension.
These anchorages use a sealing of two-constituent resin whose hardening homogeneity always depends on the quality of the mixture in situ.
In the type BBRV anchorages, the cables in bunches are protected along one part of their length by a sheath fitted with a sealing collar. The injection of sealing takes place under pressure via a special unit (injection pipe) and is carried out after the anchorage is placed in position in the cavity.
These various methods call for considerable handling operations when being placed in position.